1. Field of the Invention
The present invention relates to a urea-modified carbodiimide and a process for production thereof. More particularly, the present invention relates to a urea-modified carbodiimide which has urea bonds in the carbodiimide main chain and thereby has good compatibility with thermoplastic resins and can improve the heat resistance and hydrolysis resistance of thermoplastic resins, as well as to a process for producing said carbodiimide.
2. Description of the Prior Art
Polycarbodiimides have high heat resistance and high reactivity with active hydrogen group, and polycarbodiimides of powdery form, for example, are used as a heat resistance or hydrolysis resistance improver for polyester resins. With respect to examples of application of polycarbodiimides, Japanese Patent Publication No. 15220/1963 discloses a method for improvement of hydrolysis resistance of polyester by addition of aromatic polycarbodiimide compound thereto, and Japanese Patent Application Kokai (Laid-Open) No. 5389/1971 discloses a method for improvement of hydrolysis resistance of polyester by addition of aromatic biscarbodiimide compound thereto.
The above methods for improvement of hydrolysis resistance of polyester utilize a reaction between the carbodiimide group of polycarbodiimide and the active hydrogen of active-hydrogen compound. In the reaction, the terminal carboxyl group of polyester (this carboxyl group is believed to undergo hydrolysis by its self catalysis) is blocked with a carbodiimide compound.
Aromatic polycarbodiimides, when added to a polyester resin of high polymerization degree and high melting point (e.g. a polyethylene terephthalate of industrial use), cause, owing to the high reactivity, a side reaction such as dimerization of carbodiimide group. It invites a reduction in properties, associated with gelling and, moreover, fails to achieve sufficient blocking of terminal carboxyl group, and the resulting improvement in hydrolysis resistance is very small. Therefore, said side reaction must be suppressed, for example, by introducing a group having a steric hindrance effect, to prevent the dimerization of carbodiimide group, as disclosed in the above-mentioned Japanese Patent Publication No. 15220/1963.
Aromatic polycarbodiimides have also a problem in that, when added at high temperatures, they evaporate the component isocyanate and/or aromatic amine, adversely affecting the workers and the working environment.
It is thought that the above-mentioned problems of aromatic polycarbodiimides can be alleviated by the use of aliphatic polycarbodiimides. However, aliphatic polycarbodiimides show no sufficient addition effect to thermoplastic resins for the reasons of, for example, low reactivity as compared with aromatic polycarbodiimides; moreover, aliphatic polycarbodiimides generally have inferior compatibility with resins such as polyester.